This invention relates in general to sewing machines and in particular to a new and useful machine with a trimming device for the sewing material having a trimming knife which is in operative connection with a separate drive connected with the main shaft.
A sewing machine with a trimming device is known from German Pat. No. 23 57 605. The trimming device of the sewing machine has a two-arm swing lever whose one arm is connected over an eccentric bar with an eccentric secured on the arm shaft, and whose other arm is connected over a transmission element secured thereon and a guide rod with a vertically guided trimming knife. In the transmission element, which is in the range of the axis of rotation of the swing lever, the trimming knife is raised into rest position. But when the transmission element is at the end of the swing lever, the trimming knife is lowered into operating position, and thus into the range of a counterknife. In this position, a bolt, which connects two tie rods articulated on the transmission element with a swivelling coupling part, locks into a recess an entrainer part which is secured at one end of a connecting rod. The other end of the connecting rod bears on an electromagnetically releasable ratchet. Behind the ratchet is arranged a wobble plate which is driven by a separate motor.
When the trimming device acts during the sewing, the trimming knife driven by the eccentric and the needle work in synchronism. In order to prevent the timming knife from being in the way when the workpiece advances, the eccentric is so designed that it moves the trimming knife downward only during the feeding intervals of the workpiece to carry out a trimming stroke.
In order to trim the edge of the workpiece in the corner range during the rotation of the workpiece about the needle axis when the sewing machine stands still, the ratchet is unlocked and at the same time the separate motor is started so that the connecting rod pressed by spring force onto the rotating wobble plate is set into a reciprocating motion. This motion is transmitted directly to the transmission element so that the trimming knife moves up and down. But the edge of the workpiece is only trimmed when the sewing machine has first been stopped with the eccentric in a trimming position. The arm carrying the swing lever is therefore in its lower swivel position, and the trimming knife moves up and down in the range of the counterknife. In order to be able to turn the workpiece properly in the corner region the needle must be in its bottom position with the sewing machine standing still, so that it can act as an axis of rotation during the rotation of the workpiece.
But these different prerequisites for trimming, with the sewing machine running and standing still, can only be met if the known trimming device on a sewing machine is used with a bottom transport, because only in this type of sewing machine is the needle in its bottom position during the feeding interval of the workpiece, and the eccentric of the trimming device is necessarily in a trimming position with the sewing machine stopped in the bottom position of the needle.
But the known trimming device can not be used for sewing machines with bottom and needle transport, because in this type of sewing machine, the needle serving, among other things, as a feeding means, is not in the bottom position during the feeding interval, but during the feeding movement of the workpiece. If one wanted to us the known trimming device on a sewing machine with under--and needle transport and effect the trimming in the corner region of a workpiece with the sewing machine standing still and the needle in bottom position, the eccentric of the trimming device would be turned by 180.degree. relative to the position of sewing machines with a bottom transport alone, so that the trimming during the sewing would necessarily take place during the feeding movement of the workpiece. In this case the workpiece moving relative to the trimming knife would stall at the trimming knife and thus be moved out of the desired feeding direction.